Just to expand on Andy's post In RAID0 (a striped array), if you have an 80gb and a 120gb, the array will only be 2x80 or 160gb but benefits from increased performance. In JBOD (just a bunch of disks) you would have 200gb but the performance would be no better than a single sata on its own. JBOD basically just takes disks of any size and makes them into one large drive.

It would be done in the RAID configuration at the BIOS level. Bear in mind any disks you use will lose all data on them, so don't use disks you already have data on. The configuration software on the card/motherboard should warn you of this too.

I don't recommend you use RAID0 or RAID1 for storing betas. If you lose one disk you lose everything in the array with RAID0 and with RAID1 its a waste of a disk. If you can go for RAID5 which is n-1 in drive space, n being the number of disks. Its redundant so any single disk can fail without losing any data. However motherboard style setups won't allow dynamic disk expansion, so when you make the array be sure its the size you want to keep it. If you want to change it later it will lose all your data. That is of course unless you want to fork out over £200 on a low end RAID card